Hi Thomas,

I can't speak to your plywood questions. Rgearding the fiberglass, the ounce weight is per square yard. It is my understanding (new builder here) that the glass will take about the same weight per square yard in epoxy when finished. In other words, 6 ounce FG will weigh 12 ounces per square yard after all fill coats of epoxy. So, multiply your outside square yards by 2X the glass weight to get an approximation of the total exterior glass and epoxy weight. A typical kayak of 18 ft length is approximately 6 square yards exterior, so 6 ounce glass would yield 6 x 12 = 72 ounce (4.5 pounds) glass and epoxy exterior weight. The interior would be the same except you do not always use fill coats of epoxy, so the weight should be a little less.

Barry